U.S. patent application number 15/919317 was filed with the patent office on 2018-09-13 for method for testing a container for tightness.
The applicant listed for this patent is Robert Bosch GmbH. Invention is credited to Lucien Matthias Drescher, Wolfgang Maurischat, Thomas Peter, Wolfram Schindler.
Application Number | 20180259421 15/919317 |
Document ID | / |
Family ID | 63258912 |
Filed Date | 2018-09-13 |
United States Patent
Application |
20180259421 |
Kind Code |
A1 |
Schindler; Wolfram ; et
al. |
September 13, 2018 |
METHOD FOR TESTING A CONTAINER FOR TIGHTNESS
Abstract
A method for testing a container (11) having an interior volume
(12) for tightness, comprising the following steps: providing the
container (11) in a pressure chamber (10) and reducing pressure
within the pressure chamber (10) or increasing pressure within the
container (11) as far as a predefined test pressure, ascertaining a
pressure profile (100) within the pressure chamber (10) over time,
comparing the pressure profile (100) to a reference profile (200)
in order to determine whether a leak is present within the
container (11), wherein ambient conditions of the container (11)
and/or the pressure chamber (10) are monitored and the reference
profile (200) is adapted to changing ambient conditions.
Inventors: |
Schindler; Wolfram;
(Kirchberg, DE) ; Maurischat; Wolfgang;
(Satteldorf, DE) ; Peter; Thomas; (Kloten, CH)
; Drescher; Lucien Matthias; (Fichtenau, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Robert Bosch GmbH |
Stuttgart |
|
DE |
|
|
Family ID: |
63258912 |
Appl. No.: |
15/919317 |
Filed: |
March 13, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01M 3/3254 20130101;
G01M 3/3245 20130101; G01M 3/3263 20130101; G01M 3/329
20130101 |
International
Class: |
G01M 3/32 20060101
G01M003/32 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2017 |
DE |
10 2017 204 102.1 |
Claims
1. A method for testing a container (11) having an interior volume
(12) for tightness, comprising the following steps: positioning the
container (11) in a pressure chamber (10) and reducing pressure
within the pressure chamber (10) or increasing pressure within the
container (11) to a predefined test pressure, ascertaining a
pressure profile (100) within the pressure chamber (10) over time,
comparing the pressure profile (100) to a reference profile (200)
in order to determine whether a leak is present within the
container (11), monitoring ambient conditions of the container (11)
and/or the pressure chamber (10); and adapting the reference
profile (200) to changing ambient conditions.
2. The method according to claim 1, characterized in that adapting
the reference profile (200) is effectuated by ascertaining the
pressure profile (100) in the case of at least one reference
container with a predefined opening and/or a reference body and by
setting the pressure profile (100) as a new reference profile
(200).
3. The method according to claim 1, characterized in that adapting
the reference profile (200) is effectuated by calculating a new
reference profile (200) from an obsolete reference profile (201) on
the basis of a modification of a parameter of the ambient
conditions.
4. The method according to claim 3, characterized in that the new
reference profile (200) is calculated from the obsolete reference
profile (201) by a point-wise transformation and/or interpolation
and/or is determined by a selection from a library with
predetermined reference profiles (200).
5. The method according to claim 1, characterized in that peaks in
the pressure profile (100) are identified and removed from the
pressure profile (100) and/or the associated measurement is
discarded and repeated, wherein a peak corresponds to a measurement
value of the pressure profile (100) which, in comparison with
further measurement values, has a dispersion that exceeds a
predetermined value.
6. The method according to claim 1, characterized in that faulty
seals of the pressure chamber (10) are identified by virtue of
steps in the pressure profile (100) being ascertained and/or slopes
of a plurality of ascertained pressure profiles (100) having a
dispersion which exceeds a predetermined value.
7. The method according to claim 1, characterized in that liquid
leaks are identified by virtue of the pressure profile (100)
exhibiting a curve which, at least in part, asymptotically
converges to a saturation vapor pressure, known in advance, of a
liquid emerging from the liquid leak.
8. The method according to claim 1, characterized in that only
linear portions of the pressure profile are taken into account in
the comparison step.
9. The method according to claim 1, characterized in that a
differential pressure curve between a reference line (8) and a
measurement branch (7) connected to the pressure chamber (10) is
determined by way of a differential pressure sensor (6), wherein
both the measurement branch (7) and the reference line (8) are
blocked by way of a measurement valve (5) in order to undertake a
fine measurement of the pressure profile, and wherein a start of
the fine measurement is assumed as such a point within the
differential pressure curve at which the differential pressure
curve and/or a slope of the differential pressure curve has a
discontinuity.
10. A non-transitory machine-readable storage medium having a
computer program with program code for carrying out the method
according to claim 1 when the computer program is executed on a
computing apparatus.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for testing a
container for tightness. Moreover, the invention relates to a
corresponding computer program product.
[0002] The method of testing tightness with positive pressure or
negative pressure ("vacuum") is applied as a technical method
within quality control of pharmaceutical containers in particular
and closed containers in general. Particularly within the scope of
primary packaging of medicaments, testing tightness by means of a
vacuum is a fundamental nondestructive testing method because it is
universally applicable.
[0003] To this end, a container (vial, ampoule, etc.) that was
filled and sealed under atmospheric pressure (or a partial vacuum)
is placed in a vacuum in a closed chamber during the testing
process; i.e., the space around the container is evacuated and then
locked by a valve. Subsequently, a potential pressure change within
a certain test time is measured. If the pressure remains constant,
this indicates the tightness of the chamber and the inserted
container. If the pressure increases during the test time, this
indicates a leaky container in the case of a tight test system. The
gaseous content (usually air) flows out through leaks such as
holes, tears, foreign bodies on the plug, etc., until the pressure
between the interior of the container and the vacuum chamber has
equalized. In the case of a liquid content, emerging liquid can
likewise be detected as a pressure increase if the vacuum in the
test chamber lies under the saturation vapor pressure (23 mbar for
water at 20.degree. C.) such that the liquid boils.
[0004] A similar functionality, which is virtually inverted to the
vacuum testing, is the tightness test by means of positive
pressure. The advantage herein lies in the greater applicable
pressure range. However, a disadvantage consists of the potential
risk of leaks (e.g. tears) being sealed by the positive pressure.
Moreover, already damaged containers run the increased risk of
bursting under the higher pressure difference. Moreover, the method
is not suitable for identifying leaks due to liquid (test above
saturation vapor pressure and pressure difference from outside to
inside).
[0005] For this fundamental type of tightness test based on
pressure changes, there are a number of patent applications,
specialist publications and standards. The above-described
procedure is prior art and disclosed.
[0006] Currently, the following evaluations are conventional in the
case of tightness tests based on a vacuum or positive pressure.
Thus, the integral pressure change .DELTA.p (in Pa) from start to
end of the measurement phase with the time duration t, or the
pressure increase over time .DELTA.p/.DELTA.t (in Pa/s), is
evaluated in e.g. the ASTM F2338-09 standard and in the publication
Wolf et al., Vacuum Decay Container/Closure Integrity Testing
Technology. Part 1. ASTM F2338-09 Precision and Bias Studies. PDA J
Pharm Sci Technol 2009, 63, 5, 472-488, and in the tightness
testing appliance CETATEST515 by CETA Testsysteme GmbH.
[0007] The leakage rate Q derived from the pressure measurement is
the pV throughput per unit time of a specific fluid (in this case
air) through a leak under certain conditions, usually at a pressure
difference of 1013 mbar and a temperature of 20.degree. C. ("helium
standard conditions"):
Q=.DELTA.(pV)/.DELTA.t(amount of gas per unit time [Q]=mbar1/s)
(Formula 1)
[0008] In the tightness test, the essential volume is the residual
volume V.sub.R, which corresponds to the chamber volume (including
possible channels up to the pressure sensor) minus the volume of
the test object.
[0009] If the leakage rate is expressed as a volumetric flow rate
in standard ml/min, the unit is related to normal conditions (DIN
1343: 0.degree. C., 1013 mbar). The reference to a standard
reference atmosphere (DIN 1945-1: 20.degree. C., 1 bar, dry air) is
also conventional.
[0010] A stable measurement state is rendered more difficult by
outgassing or temperature effects during the evacuation and by
subsequently flowing air from capillaries such as e.g. threads.
Such effects are reduced by a stabilization phase between the
evacuation of the chamber and the measurement phase.
[0011] On account of environmental factors such as humidity,
temperature and atmospheric pressure, even the pressure profile of
a tight container may vary between a plurality of measurements.
Connected therewith are variations of a tightness reference value
determined therefrom and of an acceptance criterion (good/bad
criterion), which lies at a certain distance above the tightness
reference value. For the purposes of compensating such drifts, U.S.
Pat. No. 6,202,477 relating to an automated vacuum tightness test
has disclosed the method of a "dynamic reference signal". Here, the
tightness reference value is formed continuously by averaging the
pressure changes of the last n test objects evaluated to be good
and the acceptance criterion ("reference signal") is updated. An
additional, fixed upper limit prevents variations of the reference
signal that are too great.
[0012] In addition to the evaluation of the pressure increase
("fine leak test"), the tightness test process usually contains an
additional "coarse leak test", carried out beforehand and/or
afterward, as disclosed in e.g. US008544315B2. In so doing, the
measurement chamber is unified with a second volume (e.g. the rear
space of a valve), in which a deviating known pressure level is
set. From knowledge of the initial pressures and the equalization
pressure after the volume unification, an evaluation is carried out
as to whether the container has such a large leak that the interior
of the container was already completely evacuated during the
evacuation phase and it was no longer possible to identify a fine
leak due to lack of pressure difference in relation to the
measurement chamber.
[0013] The possibility of detecting liquid leaks by means of a
pressure increase test is disclosed in U.S. Pat. No. 8,55,4315B2
and further patent documents listed therein.
SUMMARY OF THE INVENTION
[0014] The method according to the invention allows a test for
tightness to be carried out within a short period of time and with
high sensitivity and repeatability. In particular, the method can
be automated with little outlay. The method according to the
invention for testing a container having an interior volume for
tightness, comprising the following steps: initially, there is a
provision of the container in a pressure chamber. Here, the
container within the pressure chamber should be examined in terms
of its tightness. If the container is tight, the interior volume of
the container is separated from the surroundings of the container.
If this is not the case, the container is leaky. In order to
identify such leakiness, there is a reduction of pressure within
the pressure chamber or an increase of pressure within the pressure
chamber as far as a predetermined test pressure. Hence, a pressure
difference arises between the interior volume of the container and
the surroundings of the container that correspond to the interior
of the pressure chamber. The container is leaky should these
pressures equalize. Therefore, an ascertainment of a pressure
profile within the pressure chamber over time and a comparison of
the pressure profile to a reference profile are effectuated.
Whether a leak is present in the container is determined in this
way. This is the case, in particular, if the pressure profile
corresponds to the reference profile and the reference profile
corresponds to the pressure profile that occurs in the case of a
leaky container. As a consequence, it is possible, in addition to
purely determining leakiness, also to make a statement about a
degree of the leakiness by virtue of the pressure profile
preferably being compared to a multiplicity of different reference
profiles. The pressure chamber is preferably a sealed chamber
within which any pressure can be set. Should the pressure be set to
a constant value, a change in the pressure in the chamber can only
be effectuated by a leak in the container. In order to eliminate
influences on the pressure profile which are not generated by a
leak of the container but which have external influences, provision
is made for the ambient conditions of the container and/or the
pressure chamber to be monitored and the reference profile to be
adapted to changing ambient conditions. The ambient conditions are,
particularly advantageously, an ambient temperature and/or an
ambient pressure and/or an ambient humidity. Since a change in
these parameters may lead to a modified pressure profile, provision
is made for also adapting the reference profile to the modified
ambient conditions.
[0015] This achieves positive and reliable identification of leaks
within the container to be tested.
[0016] In general, within the scope of this application, provision
is made for tightness to be understood to mean that the interior
volume of the container is separated from surroundings by the
container. Should this not be the case, the container has a leak
which allows the passage of a mass flow between surroundings and
interior volume.
[0017] Preferably, provision is made for adapting the reference
profile to be effectuated by ascertaining the pressure profile in
the case of at least one reference container with a predefined
opening and/or a reference body and by setting the pressure profile
as new reference profile. The predefined opening consequently
represents a deliberate leak for ascertaining the pressure profile
in the case of this deliberate leak. Here, in particular, the
pressure profile for such a reference container, which can then be
used as a reference profile, is ascertained under the same ambient
conditions as the actual testing of the container. Consequently,
the reference profile, which corresponds to the test profile of the
reference container, and the test profile during the actual testing
of the container have the same parameters. This means that both
profiles were ascertained under the same ambient conditions.
Ascertaining the pressure profile of the reference container is
preferably repeated periodically. Moreover, pressure profiles of
different reference containers are preferably ascertained in order,
as a consequence, to be able to identify and characterize different
leaks in the container to be tested. Preferably, provision is
moreover made for a regularly ascertained reference profile to be
used for self-monitoring of the testing method.
[0018] Advantageously, adapting the reference profile is
effectuated by calculating a new reference profile from an obsolete
reference profile on the basis of a modification of a parameter of
the ambient conditions. This means that physical laws are used to
calculate how the reference profile changes when there is a change
in a parameter which at least partly describes the ambient
conditions. Such a parameter can preferably be a temperature, a
pressure or a humidity. An obsolete reference profile should be
understood to mean that the latter is based on parameters of the
ambient conditions which are no longer current.
[0019] Particularly advantageously, the new reference profile is
calculated from the obsolete reference profile by a point-wise
transformation and/or interpolation. This means that individual
values of the reference profile which use an obsolete ambient
parameter are converted into new values on the basis of an updated
ambient parameter. Subsequently, there preferably is an
interpolation between the converted points. Alternatively, or
additionally, there is a selection from a library with
predetermined reference profiles. In particular, a multiplicity of
reference profiles, which are valid for a respective combination of
different ambient parameters, are stored in this library. If no
reference profile should be available for a current ambient
parameter, there preferably is an interpolation between reference
profiles for known ambient parameters.
[0020] Preferably, provision is made for disturbances in the
pressure profile to be identified and eliminated or for the
affected measurement to be discarded and repeated. To this end, in
particular, peaks in the pressure profile are identified and
removed from the pressure profile where possible. In particular,
such a peak corresponds to a measurement value of the pressure
profile which, in comparison with further measurement values, has a
dispersion that exceeds a predetermined value. In this way, the
quality of the pressure profile can be improved since signal
disturbances have no effect, or only a reduced effect, on the
method according to the invention.
[0021] Advantageously, faulty seals of the pressure chamber will be
identified. In particular, this is effectuated by analyzing the
pressure profile. To this end, steps in the pressure profile are
ascertained and/or there is an identification of slopes of a
plurality of ascertained pressure profiles having a dispersion
which exceeds a predetermined value. A step in the pressure
profile, in particular, allows identification of a non-ideal
interaction between O-ring seal and associated groove, meaning that
there is no immediate complete sealing effect. By identifying
faulty seals, it is possible, in particular, to output a warning
signal that the identification of leakiness of the container may be
faulty since the seal of the pressure chamber is possibly
defective.
[0022] Moreover, provision is made in a further preferred
embodiment for liquid leaks to be recognized and identified as
such. In particular, a liquid leak is identified by virtue of the
pressure profile initially exhibiting a strongly increasing curve,
which asymptotically converges to a saturation vapor pressure,
known in advance, of the liquid. To this end, provision is made for
the predefined test pressure to lie below the saturation vapor
pressure known in advance. By way of such a characteristic profile,
it is possible to distinguish between air leaks and liquid leaks.
If a plurality of different leaks are present, particularly a
liquid leak and an air leak, the pressure profile exhibits a
corresponding superposition of the characteristic profiles for the
different leaks. In particular, the curve of the liquid leak with
asymptotic convergence is superposed on a linear curve of the air
leak. The superposed individual partial curves of the pressure
profile can be extracted easily and with little outlay from the
pressure profile. Preferably, the pressure profile exceeding a
threshold at the start of the fine leak test is a first criterion
to reject the container to be tested as leaky on account of a
liquid leak or a large air fine leak.
[0023] Preferably, only linear portions of the pressure profile are
taken into account in the step of comparison with one or more
reference profiles. Since the small air leaks to be identified in
this step, in particular in the single-digit micrometer range, lead
to a linear pressure profile during the test time with a negligible
effect on the pressure difference between container interior and
pressure chamber, this ensures that only relevant sections of the
pressure profile are used for identifying leakiness. Since the
other regions of the pressure profile do not signal leakiness, a
comparison with the reference profile which has a characterization
of the leakiness as a content can be avoided in these regions.
Consequently, the method can be carried out in a fast and
resources-sparing manner.
[0024] Preferably, a differential pressure curve between a
reference line and a measurement branch connected to the pressure
chamber is determined by way of a differential pressure sensor.
Here, both the measurement branch and the reference line are
blocked by way of a measurement valve in order to undertake a fine
measurement of the pressure profile. The measurement valve is
preferably a double valve. The reference line is preferably only
provided for connecting the differential pressure sensor and the
measurement valve such that there is a reference for the pressure
within the pressure chamber. A start of the fine measurement is
assumed as such a point within the differential pressure curve at
which the differential pressure curve and/or a slope of the
differential pressure curve has a discontinuity. If there is
pressure equilibrium in the pressure chamber and in the reference
line before the measurement valve is closed, the pressure increases
in the pressure chamber after the measurement valve is closed. In
practice, this also occurs in the case of completely sealed
containers within the pressure chamber, for example on account of
residual leakiness of the pressure chamber. The onset of the
increase is consequently an indication for a completely closed
measurement valve and, as a result, for the start of the fine
measurement. If a residual gas amount is left in the reference line
or in the pressure chamber, there is a compression or relaxation
when the valve of the measurement valve is closed, which is
identifiable from a discontinuity in the differential pressure
profile. As a consequence, it is possible to identify the time at
which the valve of the measurement valve is completely closed in a
simple manner and with little outlay, but also positively and
reliably at the same time, and hence it is possible to accurately
determine the start of the fine measurement.
[0025] The invention moreover relates to a computer program product
having program code stored on a machine-readable storage medium.
The program code serves to carry out the method as described above
when the computer program product is executed on a computing
apparatus. In particular, the computing apparatus can be a computer
and/or a controller.
[0026] In conclusion, the invention allows a fully automatic
pressure increase test with cycle times of a few seconds with, at
the same time, a high sensitivity and reproducibility over a time
period of several hours. In order to meet these requirements, the
advantageous properties set forth below emerge.
Identification and Computational Compensation of Pseudo-Leakage
from Virtual Leaks
[0027] If a pressure increase is measured in the tightness test on
a vacuum basis, this does not necessarily indicate a leaky test
object, i.e. a leakage flow from the interior of the container.
[0028] Pseudo-leakage from virtual leaks, i.e. pressure increases
which occur in a real measurement system, such as residual
leakiness, subsequent flow from threads, settlement movements of
seals and reactions to humidity (reception in an open chamber and
subsequent outgassing under a vacuum), can be superposed on the
actual measurement signal and appear as a leak. Such effects can be
minimized by structural measures and/or by subtracting a tight
reference profile (pressure curve for a tight reference body, in
particular a tight reference container, in a tight test
chamber).
[0029] Moreover, the present invention preferably facilitates the
identification and computational compensation of pseudo-leakage
which emanates from the test objects themselves.
[0030] Such effects may emanate, in particular, from cavities on
the outside of the containers, from which the air (at atmospheric
pressure) does not completely escape during the evacuation phase.
By way of example, vials have air inclusions behind the flanged cap
(see FIG. 1). Through gaps, the air still creeps into the test
chamber during the measurement phase and leads to a pressure
increase there. Here, the assumption must be made that vials from
the same production batch exhibit differently sized virtual leaks,
i.e. the effect must be considered individually for each vial.
Dynamic Generation of the Acceptance Criterion
[0031] In the ideal case, the pressure curve of a tight reference
body in a tight pressure chamber is a constant signal
(corresponding to the pressure in the test chamber in the case of
an absolute/relative pressure measurement; ideally .DELTA.p=0 Pa in
the case of the differential pressure measurement).
[0032] In practice, the profile of pressure curves depends on
ambient conditions such as temperature (T), atmospheric pressure
(p.sub.atm) and humidity (rH). T and rH, in particular, have an
effect on the outgassing behavior of the measurement system and,
possibly, the containers to be tested, e.g. the plastic or paper
components thereof, as well, even if the measurement system has
already been optimized structurally in respect of a low outgassing
behavior under vacuum conditions.
[0033] If, as described in e.g. U.S. Pat. No. 6,202,477, the
tightness reference value and hence the acceptance criterion is
continuously formed from the trend of the last containers tested to
be tight, there is the risk of systematically occurring trends
falsifying the evaluation; for example, there is the risk that
containers with continuously increasing leakage shift the
acceptance criterion ever further upward. Therefore, such a method
is only applicable within a tightly delimited framework. The
present invention indicates a more universally applicable
method.
Reduction in the False Reject Rate
[0034] Further effects which may lead to a negative test result but
are not caused by a leaky test object include broken seals, in
particular the dynamically greatly burdened chamber seal, and
(electronic) signal disturbances. The pressure curves should be
specifically analyzed for these, in order to treat such effects
separately. If the measurement valve switches with undefined
delays, the pressure curves should be aligned to a temporal
reference point prior to the evaluation on the basis of
characteristic features in the curve.
Predictive Maintenance
[0035] Going beyond the improvement in the test quality, it is
possible to evaluate the functionality of the process valves (and
their pilot valves) on the basis of features. In addition to the
aforementioned evaluation of the chamber seal, dedicated service
measures should be proposed therefrom ("predictive
maintenance").
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] Exemplary embodiments of the invention will be described in
detail below with reference to the attached drawing. In the
drawing:
[0037] FIG. 1 is a schematic view of a container, the tightness of
which is tested using a method according to an exemplary embodiment
of the invention,
[0038] FIG. 2 is a schematic view of a multiplicity of different
test pressure profiles, as may occur when carrying out the method
according to the exemplary embodiment of the invention,
[0039] FIG. 3 is a schematic view of an adaptation of a reference
profile during the method according to the exemplary embodiment of
the invention,
[0040] FIG. 4 is a schematic view of a measurement apparatus for
carrying out the method according to the exemplary embodiment of
the invention,
[0041] FIG. 5 is a schematic view of different pressure profiles
during the method according to the exemplary embodiment of the
invention,
[0042] FIGS. 6.1-6.4 are a first schematic flowchart of the method
according to the exemplary embodiment of the invention,
[0043] FIGS. 7.1-7.5 are a second schematic flowchart of the method
according to the exemplary embodiment of the invention,
[0044] FIG. 8 is a schematic view of a curve of the derivative of
the test pressure profile during the method according to the
exemplary embodiment of the invention,
[0045] FIG. 9 is a schematic view of a point-wise adaptation of the
reference profile during the method according to the exemplary
embodiment of the invention,
[0046] FIG. 10 is a schematic view of a noise identification of a
pressure signal during the method according to the exemplary
embodiment of the invention,
[0047] FIG. 11 is a schematic view of an identification of a
defective seal of the measurement apparatus during the method
according to the exemplary embodiment of the invention,
[0048] FIG. 12 is a schematic view of an identification of a start
point of a measurement during the method according to the exemplary
embodiment of the invention, and
[0049] FIG. 13 is a schematic view of an identification of a liquid
leak during the method according to the exemplary embodiment of the
invention.
DETAILED DESCRIPTION
[0050] The embodiment described below is directed to, in
particular, the pressure increase test (testing tightness in a
vacuum) for identifying leaks. Equally, the explanations below, in
principle, can be applied--with an inverted sign--to tightness
tests on a positive pressure basis.
Overview of a Preferred Embodiment of the Invention
[0051] FIG. 1 schematically shows a container 11 having an interior
volume 12. The interior volume 12 is delimited from surroundings by
the container 11, which has been provided with a lid 13. However,
the container 11 may have leaks which can lead to leakage. This is
indicated schematically in FIG. 1 by a first volumetric flow rate Q
and a second volumetric flow rate Q.sub.A.
[0052] The first volumetric flow rate Q is a volumetric flow rate
from the interior volume 12 to the surroundings. Consequently, the
container 11 has a leak. By contrast, the second volumetric flow
rate Q.sub.A is a volumetric flow rate from a volume V.sub.A of the
container 11 which does not correspond to the interior volume 12.
In particular, this is a volume within the lid 13. Consequently,
the second volumetric flow rate Q.sub.A does not indicate a leak of
the container 11 and it should be distinguished from the first
volumetric flow rate Q. Therefore, the assumption is made below
that the first volumetric flow rate Q indicates a leak and the
second volumetric flow rate Q.sub.A indicates a virtual leak.
Identification and Computational Compensation of Pseudo-Leakage
from Virtual Leaks
[0053] FIG. 2 shows the pressure curves for a tight container, a
real leak and a virtual leak and the superpositions thereof. For
leaks that should conventionally be detected in the micrometer
range and for interior volumes 12 in the milliliter range, a real
leak leads to a linear pressure increase during the observed first
seconds. If, like in the case of the flange, the outer cavities are
small in relation to the interior volume 12 of the container and if
the gap width is large in relation to the real leak to be detected,
the curved pressure profile of the virtual leak can be separated
from the measured overall pressure increase by way of a physical
model indicated in Formula 3. Hence, the stabilization time,
otherwise required, for suctioning the external cavities empty
becomes obsolete.
[0054] FIG. 2 shows a multiplicity of different test pressure
profiles 100. A normal test pressure profile 106 sets in if no leak
is present, while a first leak profile 101 shows a pressure profile
in the case of a real leak and a second leak profile 102 shows a
pressure profile in the case of a virtual leak. Moreover, various
test pressure profiles 103, 104, 105 are shown. The first test
pressure profile 103 sets in if the container 11 has a real leak.
In particular, this curve is a superposition of the normal test
pressure profile 106 and the first leak profile 101. The second
test pressure profile 104 sets in if the container 11 has a virtual
leak. In particular, this curve is a superposition of the normal
test pressure profile 106 and the second leak profile 102. Finally,
the third test pressure profile 105 sets in if the container 11 has
a virtual leak and a real leak. Consequently, this curve is, in
particular, a superposition of the normal test pressure profile
106, the first leak profile 101 and the second leak profile 102. In
the shown diagram, the ordinate axis represents a pressure axis and
the abscissa axis represents a time axis.
[0055] The advantage of the method according to the exemplary
embodiment lies in a greater selectivity between tight and leaky
containers with, at the same time, short cycle times.
[0056] The proposed evaluation method can be applied both to
individual containers and, in the case of an appropriate extension
of the model, to the test of container groups (a plurality of
containers in the same measurement chamber).
Dynamic Generation of a Reference Profile
[0057] The acceptance criterion for the good/bad evaluation is
expediently predetermined in relation to defined conditions, for
example expressed as a maximum acceptable leakage rate in standard
ml/min or as an equivalent diameter of an ideal hole. The
corresponding pressure profile--the reference profile 200 (see FIG.
3)--or an acceptance value derived therefrom (e.g. slope in Pa/s
within a defined interval) is preferably produced by measuring a
reference container with a defined leakage rate (for example
configured as an empty product container with an inserted
calibrated orifice).
[0058] The specified ambient conditions are preferably captured by
way of appropriate sensors 14 (see FIG. 4). If at least one
threshold is exceeded, such as, in particular, the change in
temperature since the last re-examination s_.DELTA.T>1K, a new
generation of the reference profile is triggered. Alternatively, or
additionally, the trigger is effectuated if the mean of the last n
measurements exceeds a threshold s_m that lies below the acceptance
criterion.
[0059] In a known apparatus, gaps can automatically be introduced
into the supply in a wanted and programmable manner, said gaps then
being equipped with reference containers ("re-examination
containers" in that case). If no automatic supply is available, the
test machine can alternatively request a certain reference
container within a predetermined time interval for manual placement
into the supply (e.g. on the rotary table) and said reference
container can be identified by way of an appropriate apparatus such
as e.g. a barcode or RFID reader.
[0060] An alternative to the use of reference containers, or as a
complement in the case of a selection of long time intervals
between reference measurements, the reference profile 201, or the
evaluation variables derived therefrom, can be transformed to the
current ambient conditions by way of physical relationships, as
shown schematically in FIG. 3. In principle, the transformed
reference profile 200, which is obtained by means of one or more
defined reference containers and/or according to known physical
relationships, or the evaluation variables derived therefrom should
be preferred to a reference signal that is based on measurements of
the test objects themselves (like in U.S. Pat. No. 6,202,477).
[0061] FIG. 3 shows a reference profile 200 which is calculated
from a reference profile 201 that was used in preceding
measurements. In the shown diagram, the ordinate axis represents a
pressure axis and the abscissa axis represents a time axis.
[0062] The advantage of the methods, described below, for
dynamically generating the reference profiles 200 lies in the
increase of the reproducibility, in particular during long-term
operation, in the case of varying ambient conditions and in a
shorter running-up time of the test machine (when an equilibrium
sets in between the changing exposure of the chamber to the
surroundings and vacuum).
[0063] Moreover, identifying virtual leaks requires the pressure
curve that is to be fitted to a physical model to be cleaned to
such an extent that the latter only depends on real and virtual
leaks. Therefore, the tight reference profile must be subtracted
from the measurement curve in advance.
Identifying Disturbances in the Pressure Signal
[0064] A noisy pressure signal and hence a noisy test pressure
profile 100 generally leads to a greater dispersion of the slope
values calculated from the curve (e.g. by way of regression) and
reduces the sensitivity of the test. Especially non-uniform noise
and outliers like in FIG. 10 may moreover falsify the calculated
pressure increase. In particular, a downward outlier would simulate
a lower pressure increase or a lower leakage rate, which could lead
to a false positive test result. Conversely, an upward outlier
could lead to a false negative result. Therefore, signal
disturbances should be identified prior to the evaluation and the
test should be terminated with a result of non-inspected. In the
case of multiple occurrences in a test machine, the relevant test
unit should be deactivated automatically. If the fault occurs in a
plurality of test units in a test machine (sequence error or
added-up error), this is an indication for a global disturbance
source and should lead to the machine being stopped.
Identifying Faulty Seals
[0065] The valves may have an operating duration of several million
strokes and they are preferably protected from the surroundings by
means of filters. By contrast, greater wear and a higher risk of
contamination should be expected at the exposed chamber seal (e.g.
abrasion of the seal, foreign particles). Such a defective seal can
be specifically identified during the running machine operation by
way of suitable evaluations of the pressure curve of the test
pressure profile 100 and false measurements can be avoided.
Moreover, a change of the seal can be recommended in good time,
still before the effect influences the test result.
Determining the Start Time of the Fine Measurement
[0066] For the best-possible reproducible evaluation of the
pressure-time signal, the start point of the evaluation is
determined from a characteristic reaction of the pressure signal to
one or more valve switches. Switching of a process valve is
triggered in the preferred use of a pneumatic valve by way of an
electrically actuated pilot valve or it is directly electrically
actuated in the case of an electrically operated valve. Preferably,
the actuation is effectuated by a real-time system; otherwise, the
reactions to the switch in the pressure signal (actual time) may
already deviate from the intended time due to the control
electronics. In principle, differences between the actual and
intended time may occur over the entire actuation and measuring
distance, in particular as a result of delays of the process or
pilot valve (e.g. fatigued return spring).
[0067] If use is made of a differential pressure sensor 6 (cf. FIG.
4), the pressure chamber 10 (cf. FIG. 4) along with the measurement
branch 7 (cf. FIG. 4) and the reference branch 8 (cf. FIG. 4) are
preferably evacuated before the fine measurement until pressure
equilibrium prevails on both sides of the differential pressure
sensor 6, i.e. until a constant pressure signal sets in (FIG. 12;
valve closure at t4.b). Likewise, the fine measurement can be
started early from a differential pressure level that still is
elevated or reduced (FIG. 12; valve closure at t4.a), provided that
the initial level is reproducible and the remaining usable
measurement range is sufficient for evaluating the tightness.
Predictive Maintenance of the Valves
[0068] Wear can be deduced in good time from the switching times of
the valves and the reaction in the test pressure profile (e.g. time
difference, slopes deviating from reference values); consequently,
maintenance measures can be proposed and outage times can be
reduced.
[0069] The deviation of the actual switching times ascertained from
the measurement data from the intended switching points, or
statistical variables based thereon (e.g. sliding mean values,
standard deviations), of many cycles is used as an indicator for
wear/fatigue of the valves and/or the pilot valves. By way of
example, the measurement system can indicate the expected remaining
valve service life on the basis of empirical values from load tests
or it may recommend the servicing of a valve if a threshold is
exceeded.
[0070] Systematically occurring leaky results (REJECT) in a test
unit may moreover be used as an indicator for a leaky measurement
system. Whether the cause lies in the chamber seal or the valves
can be effectuated by way of a separate system test.
DETAILED DESCRIPTION OF THE EMBODIMENT OF THE INVENTION
[0071] 1 Measurement setup
[0072] FIG. 4 shows an exemplary measurement apparatus having the
following constituent parts: vacuum source 1, vacuum valve 2,
absolute or relative pressure sensor 3, rear space of the valve 4,
measurement valve 5, in particular as a double valve with two
mechanically coupled seats, differential pressure sensor 6,
measurement branch 7, reference branch 8, ventilation valve 9,
pressure chamber 10, container 11 to be tested. The measurement
valve 5 could also be embodied as two separate valves.
2 Overall Course of the Test Cycle
[0073] The course of an exemplary test cycle is indicated in FIG.
5. For the exemplary embodiment of the present invention, the
differential pressure-time curve 300, in particular, is of
importance, the latter being ascertained by the differential
pressure sensor 6. Moreover, a relative pressure-time curve 400 is
shown, the latter being ascertained by means of the relative
pressure sensor 3. In the shown diagram, the ordinate axis
represents a pressure axis and the abscissa axis represents a time
axis.
[0074] The test begins at a time t0 with a closed, aerated chamber
10 and an evacuated rear space of the valve 4. At t1, the vacuum
valve 2 and the ventilation valve 9 are closed. At t2, the
measurement valve 5 is opened. The signal of the absolute or
relative pressure sensor 3 is evaluated between t2 and t3 in order
to carry out a coarse leak test. At t3, the vacuum valve 2 is
opened and hence the measurement branch 7, the measurement chamber
10, and the reference branch 8 and the rear space of the valve 4
are evacuated. At t4, the measurement valve 5 is closed, the latter
preferably being configured as a double valve and simultaneously
closing the reference branch 8. The differential pressure sensor 6
now indicates the pressure increase in the measurement chamber 10
in relation to the reference branch 8. The signal is not evaluated
in a brief period of time between t4 and t5; this is, in
particular, a stabilization phase. At t5, there preferably is a
first evaluation as to whether the pressure exceeds a threshold
FL_ThMax. This would indicate a large fine leak which, however, is
smaller than a coarse leak to be determined during the coarse leak
test or a liquid leak. Between t7 and t8, at the end of the fine
leak phase, the slope m1 of the differential pressure-time signal
300, which represents the test pressure profile between t3 and t8,
is calculated as an evaluation variable for the tightness. A small
quotient of m1 and a slope m2, calculated at the start between t5
and t6, is an indication of a virtual leak. At t8, the ventilation
valve 9 is opened and hence the measurement chamber 10 and the
measurement branch 7 are aerated.
3 Program Sequence for Generating the Reference Profiles
[0075] An exemplary embodiment for generating the reference
profiles 200 for a tight reference container and a leaky reference
container is shown in FIG. 6 for an individual chamber. In process
step (PS) R2, the deviations of the ambient conditions and the mean
value of the last containers 11 tested as tight are ascertained. In
PS R3, there is the comparison with thresholds which triggers the
generation of new reference profiles 200 if exceeded and otherwise
triggers the normal product test (PS R4).
[0076] Initially, the tight reference profile 106, as baseline(t),
and the slope m_baseline are determined in the evaluation range
[t7; t8] for a tight reference container (PS R5 to PS R10).
Subsequently, a reference container with the maximum permissible
leakage rate is requested and the slope FL_maxSlope is determined
from the pressure profile thereof in [t7; t8] (PS R11 to PS
R15).
[0077] Before a fine leak evaluation, the initial variables
baseline(t), m_baseline and FL_maxSlope are still optionally scaled
for a different chamber type, such as, in particular, a group
chamber, or for other occupancies, such as, in particular, in the
case of a reference measurement with only one container in a dual
chamber, for a product test with full occupancy.
[0078] Moreover, the functionality of the test system is monitored
by means of the reference measurements. If the slope m baseline
exceeds a threshold s_baseSlope (PS R8) or if the slope FL_maxSlope
is not between s_LowAcc and s_HighAcc (PS R14), corresponding
alarms are output.
4 Program Flowchart of the Fine Leak Evaluation
[0079] The preferred exemplary embodiment of a program flowchart
for evaluating the fine leak phase for a machine with individual
chambers and group chambers is shown in FIG. 7.
[0080] The input parameters differ according to chamber type. By
way of example, the maximum accepted slope FL_MaxSlope for the
individual chamber is initialized with a higher value than for a
group chamber on account of the smaller residual volume V.sub.R,
with both values corresponding to the same maximum accepted leakage
rate (in this respect, see also 6.1b)).
[0081] The temporal adjustment of the curve (process step PS F3) is
described in more detail in point 9, checking of the signal quality
(PS F4) in 7 and the continuity of the pressure increase (PS F5) in
8.1.
[0082] For the purposes of indicating a liquid leak (PS F8) on the
basis of the saturation vapor pressure by means of a lower
threshold VSP min and an upper threshold VSP max, see 11. In this
case, the chamber is aerated after the test (cutting the pressure
difference above the leak), but it remains closed pending a manual
removal (Status FL_manualREJECT) in order to prevent contamination
outside of the chamber. The affected test station is deactivated
for further test cycles.
[0083] In the case of a bearable risk of contamination, it
alternatively would be possible (not drawn here) to continue to use
the test station, possibly after automatic measures for drying
purposes (such as e.g. continued evacuating or purging), and the
tested container or containers could be removed like in the case of
an air leak.
[0084] The elimination of virtual leakage (optionally connectable
with PS F11) is treated by PS F12 to PS F15. In order to ensure
that only virtual leakage is in fact removed by calculation, the
value FL_MaxRatio should be selected in such a way in this case
that m2/m1<FL_MaxRatio always applies for (internally and
externally) tight reference containers--the pressure-time signal of
which in a real measurement system likewise deviates from the ideal
linear profile.
[0085] For individual and group chambers, the result of a passed
fine leak test is FL_ACCEPT. The result for a failed test in an
individual chamber is FL_REJECT, i.e. the container tested to be
leaky is sorted into a corresponding reject path. In the case of a
group chamber, the result is FL_singleRETEST, i.e. the containers
are fed to subsequent individual re-examinations (after a
sufficiently long waiting time for equalizing the internal pressure
to atmospheric pressure in the case of a leak).
[0086] If the result FL_REJECT or FL_singleRETEST occurs
systematically (PS F18), for example three times in succession for
the same test station or, overall, three times more frequently than
for the other test units, the faulty station is automatically
deactivated for further test cycles after discharging the test
object. If there was a significant dispersion of the last slopes
measured with this test station despite a result of FL_ACCEPT, this
likewise indicates a faulty station, in particular a faulty seal
thereof (see 8.2).
5 Eliminating Virtual Leaks
[0087] Ideally, after subtraction of the tight reference profile,
the pressure-time measurement curve is free from virtual leaks
originating from residual leakiness of the measurement system or
outgassing (it is also possible to take account of
potential--reproducible--outgassing effects of the test objects in
the case of a suitable selection of the tight reference container,
for example likewise equipping the latter with a paper label). In
the case of an empty pressure chamber 10 or in the case of one or
more inserted tight containers, the cleaned pressure-time signal is
ideally constantly zero.
[0088] Assumptions: [0089] the real (air) leak with the leakage
rate Q causes a linear pressure profile p.sub.K(t) in the pressure
chamber 10 (due to a negligible effect on the internal pressure and
chamber pressure, Q(0)=Q(t) during the considered time interval)
[0090] this also applies if a plurality of containers are leaky in
a group test. In the case of three containers, this can be combined
to an overall leakage rate Q=Q.sub.1+Q.sub.2+Q.sub.3. [0091]
externally enclosed air with an opening to the outside but not to
the inside into the container 11 (e.g. behind the flanged cap)
causes an exponential profile as a virtual leak since the pressure
in the substantially smaller cavity volume V.sub.A reduces
significantly. The decreasing pressure difference across the
opening in turn causes a reducing flow rate.
[0092] The pressure profile p.sub.K(t) in the measurement chamber
can be represented as follows:
p K ( t ) = 1 V R Q ( 0 ) t + ( V A p atm V A + V R ) ( 1 - exp ( -
Q A ( 0 ) V A p atm t ) ) + real leak virtual leak 1 virtual leak 2
( Formula 2 ) ##EQU00001##
with time t, the known residual volume V.sub.R, the unknown real
leakage rate Q(0) to be detected, flowing out of the container
interior, the unknown volume of the external cavity V.sub.A, the
unknown virtual leakage rate from the cavity at the start of the
measurement Q.sub.A(0) and the known atmospheric pressure
p.sub.atm.
[0093] Formula 2 describes the simplified case where the pressures
behind the virtual and real leak at t=0 correspond to the
atmospheric pressure and the chamber pressure corresponds to 0 mbar
(extensions without these simplifications are likewise
representable).
[0094] The corresponding time derivative applies for the pressure
increase:
p k ' ( t ) = 1 V R Q ( 0 ) + ( Q A ( 0 ) V A + V R ) exp ( - Q A (
0 ) V A p atm t ) + real leak virtual leak 1 virtual leak 2 (
Formula 3 ) ##EQU00002##
[0095] For a long time t, the contribution of small volumes
disappears (the exponential term tends to zero), which forms a
justification for the relatively long stabilization time that is
usually employed.
[0096] If the container 11 is exposed to the vacuum for only a
short period of time t, the constant leakage rate Q(0), which is
inversely proportional to V.sub.R, can be extracted from the
exponential term (or plurality of exponential terms in the case of
the group test) (see FIG. 8). In the diagram shown in FIG. 8, the
ordinate axis represents a time derivative of the pressure and the
abscissa axis represents a time axis. A first derivative curve 111
shows the profile in the case of a real leak, a second derivative
curve 112 shows the profile in the case of a virtual leak and a
third derivative curve 113 shows a combination of virtual leak and
real leak.
[0097] Since V.sub.R is known and only Q(0) is relevant to the
tightness evaluation, a multiparameter fit to a function of the
form
a+bexp(-ct)
with Q(0)=aV.sub.R (Formula 4)
is suitable.
[0098] The Levenberg-Marquardt algorithm (LMA), applied to Formula
5 and the partial derivatives thereof with respect to b and c, is
suitable for the parameter fit. The simplex method is another
option for the parameter fit.
6 Dynamic Generation of the Reference Profiles
[0099] The reference profiles (or corresponding reference values)
can be dynamically generated using various methods, on the basis of
measurements and assisted in part by computation.
6.1 Dynamic Reference Measurement
[0100] Recording new reference profiles 200 on the basis of
suppliable reference containers can be triggered by one or more of
the following events (R2 to R3 in FIG. 6): [0101] A significant
deviation of the ambient conditions in relation to the last
reference measurement [0102] A significant deviation of the mean
pressure increase or the corresponding leakage rate of the last n
product containers evaluated as tight in comparison with the last
tight reference value
[0103] Independently thereof, a preferably time-controlled
reference measurement, particularly preferably an additional
time-controlled reference measurement, is carried out. After
recording a new reference profile with a reference container, there
optionally can be by means of at least one further reference
container a verification of the new reference measurement or of the
new acceptance criterion derived therefrom.
[0104] For the purposes of generating the reference profiles of an
individual or group chamber, the following variants are suitable,
particularly in a test running in an automated manner: [0105] a) A
tight reference profile from the measurement of the empty chamber
(i.e. format parts with a known empty volume V.sub.empty). [0106]
The multiplication of the pressure profile by the ratio of the
volume of the empty chamber to the residual volume of the chamber
V.sub.empty/V.sub.R yields the tight reference profile for a filled
pressure chamber 10. [0107] Advantage: no tight reference
containers necessary [0108] disadvantage: lower sensitivity, as
V.sub.empty>V.sub.R [0109] b) Reference measurement using
chambers with one or more inserted reference containers. [0110] If
the pressure chamber 10 is not completely filled, e.g. a singly
occupied dual chamber, multiplying the pressure curve by
V.sub.R(1)/V.sub.R(2) yields the reference curve for a completely
filled chamber. [0111] Advantage: higher sensitivity than in (a)
due to a smaller V.sub.R, the same conditions as in the test of the
containers 11 in the case of the completely filled pressure chamber
10. Precondition: possibility of supplying the reference containers
[0112] c) Reference measurement using a certain reference chamber
with an inserted reference container. [0113] In a test system
consisting of a plurality of units, for example arranged on a
carousel, the assumption can be made that changing ambient
parameters have the same effect on all chambers, possibly linked by
way of a correction function (e.g. relating to different outgassing
behavior on account of differently large surfaces of the individual
chamber and dual chamber). Then, the reference profiles can be
determined for only one of the chambers and transferred to the
other chambers. In the case of a test machine with a plurality of
test units, e.g. 4.times.(8 group chambers+1 individual chamber),
one or more individual chambers are preferably used for the
reference measurements (on account of a smaller V.sub.R and
equipping/forming a gap being simpler). [0114] Advantage: reference
measurements are only carried out for one or a few pressure
chambers 10. Therefore, reference profiles 200 can be produced more
frequently and very close to the current ambient conditions. The
reference profiles 200 need not be transformed to new ambient
conditions by calculation. [0115] Precondition: possibility of
supplying the reference containers
[0116] Preferably, the reference containers have the same
(external) volume as the containers 11 to be tested and they are
preferably free from virtual leaks. If the container 11 is a vial,
the reference container can be configured as the same vial with a
rigid lid with or without an orifice or a different opening with a
known leakage rate.
6.2 Point-Wise Transformation of a Standard Reference Curve
[0117] FIG. 9 shows an exemplary embodiment for calculating a new
reference profile 200 depending on the ambient parameters of
temperature T, atmospheric pressure p.sub.atm and relative humidity
rH. In this embodiment variant, each measurement point
p.sub.std(t.sub.1) . . . p.sub.std(t.sub.n) of an already known
standard reference profile is transformed to a current reference
profile by way of a dedicated function f.sub.1 . . . f.sub.n, which
is dependent on the ambient parameters. In the diagram shown in
FIG. 9, the ordinate axis represents a pressure axis and the
abscissa axis represents a time axis.
[0118] If the combined gas law pV/T=constant and, additionally, a
linear dependence (with slope b) of the humidity are used as a
basis, the calculation of p.sub.new(t.sub.i) is e.g.
p.sub.new(t.sub.1)=p.sub.std(t.sub.1)T.sub.new(t.sub.1)/T.sub.std(t.sub.-
1)b(t.sub.1)rH.sub.new(t.sub.1)/rH.sub.std(t.sub.1) (Formula 5)
[0119] By way of example, the parameter b(t) can be determined from
a plurality of standard reference values for different ambient
parameters by way of a parameter fit (individually for each time
t.sub.n!).
[0120] Depending on the nature of the test chamber 10 and the
container 11, the humidity intake upon contact with ambient air and
the emission under vacuum may additionally depend both on the
humidity and on the temperature. Formula 5 should be extended
accordingly.
[0121] In a further exemplary embodiment, standard reference
profiles p.sub.ref(t), which were measured under known conditions,
are initially respectively fitted using a function such as e.g. a
polynomial function of at least fourth order.
f(t)=at+bt.sup.2+ct.sup.3+dt.sup.4 Formula 6
[0122] Unlike in 6.1, it is not the measurement points of the curve
but only parameters of the fitted function f(t) that are stored in
the measurement system and used for the evaluation. The advantage
lies in less data to be stored and data that can be processed more
quickly; secondly, the reference profile is smoothed by the
fitting.
[0123] The pre-factors of the polynomial are determined in turn as
functions depending on T, p and rH from a plurality of such smooth
reference profiles (for example by parameter fitting).
[0124] Then, the reference profile for the new ambient conditions
is generated from these pre-factor functions:
p.sub.new(t)=a(T, p, rH)t+b(T, p, rH)t.sup.2+c(T, p,
rH)t.sup.3+d(T, p, rH)t.sup.4 Formula 7
[0125] In contrast to variant 6.1, this embodiment variant tends to
require fewer fit parameters and a shorter calculation time.
However, the selection of suitable pre-factor function tends to be
more difficult.
6.3 Comparison with a Library of Reference Profiles
[0126] In a further exemplary embodiment, reference profiles are
recorded in a systematic grid of defined ambient conditions, in
particular at temperatures and humidities in the range relevant to
operation (e.g.: temperature in 1.5 K steps between 15 and
30.degree. C. and, simultaneously, the humidity from 20% rH to 50%
rH in 5% rH steps). The reference profile is stored in the test
system (in the example n=60).
[0127] Using this embodiment variant, possible fitting errors as in
variants 6.2 and 6.3 are excluded as a matter of principle.
However, an expedient variant, in this case too, is that of fitting
each individual curve in advance to e.g. a polynomial function and
only storing the pre-factors.
[0128] A disadvantage lies in the necessity of a sufficiently fine
gradation and, in particular, in a corresponding outlay for
measuring the multiplicity of defined reference profiles.
6.4 Combinations of the Aforementioned Variants
[0129] The aforementioned variants can be combined with one
another. In particular, reference profiles as in 6.4 can be stored
in the memory with only a coarse gradation of the ambient
conditions (e.g. a temperature gradation of 5 K) and an
interpolation according to 6.3 could be effectuated
therebetween.
7 Identifying Disturbances in the Pressure Signal
[0130] If peaks as in FIG. 10 occur in the signal, a slope value
that is much too large or much too small might result. In order to
detect such signal disturbances, it is possible to form, in a
sliding manner over n measurement points, the standard deviation
700, which is calculated, in particular, in a sliding manner, after
a time FL_T Noise and said standard deviation can be compared to an
upper limit FL_maxNoise. In the diagram shown in FIG. 10, the
ordinate axis represents a pressure axis and the abscissa axis
represents a time axis.
8 Identifying a Faulty Chamber Seal
8.1 Multi-Step Increase in the Pressure Curve
[0131] The slope dp/dt of the test pressure profile 100 ideally
decreases between t4 and t8 and it is positive (a negative slope
would indicate leakiness of the reference branch 8 or an expansion
of the residual volume). By way of example, if the interplay
between an O-ring, the groove and the counter surface does not work
in an ideal fashion (i.e. no immediate complete sealing effect from
the chamber interior to the groove and to the outside), multi-step
pressure curves may occur, as on the left of FIG. 11. In the shown
diagram, the ordinate axis represents a pressure axis and the
abscissa axis represents a time axis. Shown therein is a section of
a test pressure profile 100 and a curve of a first derivative 110
of the test pressure profile 100.
[0132] Such steps in the increase can be identified, for example,
by calculating and analyzing piecewise slopes, as illustrated on
the right of FIG. 11 (in this case slopes of the pressure curves
illustrated on the left, calculated in a sliding manner over 21
points).
8.2 Dispersion of the Slope Values
[0133] An indication for a reducing sealing effect of the chamber,
for example by rubber abrasion or another contamination of the
sealing surface, is that the dispersion (e.g. expressed as a
standard deviation) of the slopes m1 of the last n measurements
deviates significantly from the expected value FL_MaxSlopeDev (PS
F18). This threshold preferably lies below the acceptance criterion
in order to adopt measures such as a deactivation of the chamber or
a stop for cleaning of the seal in good time before the test result
is influenced.
9 Determining the Start Time of the Fine Measurement
[0134] FIG. 12 shows typical examples of a pressure profile using a
differential pressure sensor 6. In the shown diagram, the ordinate
axis represents a pressure axis and the abscissa axis represents a
time axis. Closing the measurement valve 5 defines the start of the
fine measurement. This time can be determined from the test
pressure profile 100 as set forth below.
[0135] In a preferred embodiment variant, the slope of the test
pressure profile 100 is evaluated around the intended switching
point, e.g. in an interval +/-200 ms.
[0136] If there is equal pressure on both sides of the differential
pressure sensor 6 prior to closing the measurement valve 5, i.e. if
there is a constant zero level, the differential pressure increases
after closing the valve at the time t4.b (in practice, this even
occurs in the case of an empty pressure chamber 10 on account of
residual leakiness, outgassing, etc.). The onset of the increase
can be used as indication of the valve closure of the measurement
valve 5 and hence of the start of the fine measurement. This
becomes clear from the first differential pressure curve 500.
[0137] If the measurement valve 5 is closed still during the
pressure equalization between the measurement branch 7 and the
reference branch 8 at the time t4.a in a sub-variant, the switching
point should be selected in such a way that the slope is different
before and after the valve closure of the measurement valve 5. With
the second differential pressure curve 501, FIG. 12 shows the
critical case with a continuous transition of the curve from the
pressure equalization region (to the left of t4.a) into the fine
measurement region (to the right of t4.a).
[0138] There is another variant for identifying the valve closure
if a sufficiently large amount of gas is situated on at least one
side of the differential pressure sensor 6, the compression (or
relaxation) of said gas amount when the measurement valve 5 is
closed leading to a step in the pressure profile. This is realized
in the third differential pressure curve 502.
10 Predictive Maintenance of the Valves
[0139] In addition to the switching point of closing the
measurement valve 5, it is possible to determine further switching
points from the test pressure curve 100, it being possible to
deduce wear from the delays thereof in relation to the intended
point, said wear being expressed as sluggishness of the valves. The
following examples relate to a test cycle as highlighted in FIG.
5.
[0140] a) Opening of the measurement valve 5 [0141] The absolute
pressure signal exceeds a threshold or the differential pressure
signal drops below a threshold at the start of the coarse leak test
(t2).
[0142] b) Closing of the measurement valve 5 [0143] See 9
[0144] c) Opening of the ventilation valve 9 [0145] The
differential pressure signal exceeds a threshold at the end of the
fine leak test.
[0146] d) Opening of the vacuum valve 2 [0147] The absolute or
differential pressure drops below the threshold at the start of the
evacuation phase.
11 Identification of Liquid Leaks
[0148] In the case of a container 11 that has been partly filled
with liquid, liquid may emerge through a leak and it may vaporize
upon contact with the vacuum of the pressure chamber 10 provided
that the pressure in the measurement chamber lies below the
saturation vapor pressure 600. As illustrated in FIG. 13, an
initially steep pressure increase that tends asymptotically toward
the saturation vapor pressure 600 (fourth test pressure profile
107) is characteristic, particularly in the case of a minimized
residual volume, while air leakage in the case of conventional
headspace volumes (ml range) and leak dimensions to be tested in
the .mu.m range exhibits a virtually linear profile (fifth test
pressure profile 105). Therefore, it is possible to distinguish
between air and liquid leakage. In the shown diagram, the ordinate
axis represents a pressure axis and the abscissa axis represents a
time axis.
* * * * *